This invention relates to a system for signalling the approach of a train on a track and, more particularly, this invention is directed to a system for effecting operation of respective signalling devices at plural relatively proximate grade crossings to indicate the approach of a train. Moreover, the invention relates to a railroad signal system including a repeater receiver device between two spaced apart locations for looking through a first one of the locations to detect a train approaching both the first and the subsequent locations.
The invention will be described with reference to a system that produces a system output signal to effect pick up or dropping of the conventional railroad signal relay at a grade crossing, which will in turn effect pick up or dropping of the crossing gate, or de-energization or energization of a signal light, bell or the like, to signal street traffic whether or not a train is approaching the crossing. The railroad signal relay arrangement provides electrical isolation between the relatively low voltage railway signal system and the high voltage usually required for energization of the crossing gate operating motor, signal lamps or the like, and also provides a measure of safety whereby a failure in the railway signal system eliminating its output will result in dropping of the signal relay --i.e., failure is in what is referred to as a safe direction. However, it is to be understood that the system output signal can be used as an input to effect operation of other signalling devices, computers for monitoring, control and similar functions and the like. Moreover, although the invention will be described with reference to signalling at a grade crossing, the system also may be used for block signal control and the like.
Many of the prior art railway signal systems currently used to protect grade crossings at which a street and a railroad track cross on the same level or grade respond only to the presence of the train in the predetermined island or approach that is often required to be electrically isolated from other parts of the track by sets of insulated joints at both ends of the island or approach. These existing signal systems may respond only to a train located within the island between the transmitter and receiver tie points to the track and, thus, require long islands to provide train detection within a safe time before the train arrives at the grade crossing. Such a long island increases the difficulty of the system installation and maintenance, and such systems may effect undesirably long down times of the signal device, which is dropped when the train enters the approach or island regardless of the train speed. Other systems that respond to train approach speed, but do not include variable sensitivity features that correlate approach speed with distance from the crossing, often require plural electrical systems operating at different frequencies for achieving a minimum safe down time of the signal device.
One disadvantage with prior art railway signal systems is that without variable sensitivity, a train consisting of only a single car and/or engine may accelerate after approach time prediction to put the engine almost in the crossing before gate actuation. Another disadvantage is the relatively long ring-by time usually experienced in prior art railway signal systems, which is a nuisance to motorists. Moreover, the effectiveness of the prior art systems over a wide range of track ballast conditions is limited.
In my U.S. Pat. No. 3,850,390, issued Nov. 26, 1974, and in my U.S. Patent Applications Ser. No. 458,172, filed Apr. 5, 1974, now Pat. No. 3,929,307 and Ser. No. 568,565, filed Apr. 16, 1975, which patent and patent applications are assigned to the same assignee as the present application, are disclosed movement detector railway signal systems that include a variable sensitivity feature. The mentioned feature effects a correlation between the speed of the approaching train and the distance of the approaching train from the island defined by the system tie points to the track proximate opposite sides of the grade crossing in order that a system output signal indicative of an approaching train is produced to effect dropping of the railroad signal relay a sufficiently safe time in advance of the arrival of the train at the crossing without an unnecessarily long down time. These movement detector systems are capable of responding to the approach of a train by monitoring the dynamic affect of the approaching train on signals transmitted in the track.
The movement detector railway signal systems disclosed in my above-mentioned patent and patent applications are responsive to changes in the lumped impedance along a portion of a track, especially changes caused by a train approaching the system tie points to the track. These movement detector systems usually include a transmitter and a receiver that are coupled to the track on opposite sides of and proximate to a grade crossing, for example, so as to have a relatively short island. However, the systems are capable of transmitting their electrical signals in the track for several thousand feet in each direction from the island and, therefore, are able to look down the track in both directions to see whether or not a train has entered the monitored, and not necessarily insulated, approach. After a train has entered the approach, the systems automatically correlate the train approach speed and distance from the grade crossing so as to effect dropping of the railway signal relay at a safe, but not too advanced, time before arrival of the train at the crossing.
Each of the movement detector railway signal systems operates on AC signals, and usually successive grade crossings, for example, along a common track may be protected by different respective systems operating at different frequencies without encountering detrimental interference between the systems. When two grade crossings are proximate each other, for example, wherein the respective approaches to each overlap, it will be necessary for at least one of the movement detector systems to "look through" the adjacent crossing to detect an approaching train sufficiently in advance of its arrival at the subsequent crossing. This approach overlap is not usually a problem if the two signal systems operate on sufficiently different frequencies.
It has been found, however, that an unusually large lump of impedance may be created at a grade crossing when salt is spread to melt ice or snow on the street, for example. Such a large lump of impedance may block the signal transmitted in the track by the movement detector system connected at a proximate subsequent grade crossing, thus preventing such movement detector system from "looking through" the first-mentioned grade crossing to detect an approaching train.
The problem encountered when such a large signal blocking lump of impedance occurs may be a too short warning time at the subsequent crossing, for example, assuming a first crossing and a second or subsequent crossing are located one thousand feet apart along a railroad track and trains run on the track often at speeds of 60 miles per hour 100 kilometers per hour) only in one direction, whereby they would arrive at the first crossing before arriving at the second. A first movement detector system having its transmitter and receiver connected to the track on opposite sides of the first crossing protects the same by effecting dropping of the railroad signal relay thereat a safe time prior to the arrival of an approaching train at such first crossing, and a second movement detector is similarly coupled to the track at the second crossing for protection of the second crossing. In order for the second movement detector system to provide more than an eleven or twelve second warning time prior to the arrival of the approaching train at the subsequent crossing, it is necessary for the second movement detector system to transmit effectively its signal beyond the first crossing. Although under normal track ballast conditions there is usually no problem for the second movement detector system to look through the firsts crossing, an unusually large lump of impedance at the first crossing substantially blocking the signal of the second movement detector system from passing therethrough would permit the second system to detect the train only after it had passed the first crossing resulting in a too short 11 or 12 second warning prior to arrival of a train approaching at a speed of sixty miles per hour.